Testing apparatus for digital telecommunication

ABSTRACT

The invention relates to a testing apparatus ( 21 ) comprising means for receiving signals in a frequency range comprising a plurality of channels. The testing apparatus further comprises means for scanning said frequency range ( 210, 213 ), means for detecting at least one channel comprising a digital signal ( 211 ), means for measuring a bit error rate of said digital signal ( 212 ), means for performing at least one comparison between said bit error rate and at least one threshold ( 213 ), and means for determining a quality of said channel on the basis of said comparison.

FIELD OF THE INVENTION

The present invention relates to a testing apparatus for digitaltelecommunication.

The present invention also relates to a method of determining a qualityof at least one channel in a digital telecommunication and a computerprogram for implementing this method.

The present invention also relates to a telecommunication networkcomprising such a testing apparatus.

The present invention is particularly relevant for a testing apparatusfor digital terrestrial television, such as a testing apparatus forDVB-T (DVB-T stands for Digital Video Broadcasting-Terrestrial).

BACKGROUND OF THE INVENTION

In digital telecommunications, digital signals are sent by emitters andreceived by a user, for example by means of an antenna. It is oftenuseful to know the quality of the received digital signals. For example,the quality of the received signals can be used in order to orientatethe antenna adequately. Moreover, if the digital signal represents anaudiovisual content, it might be useful for the user to know the qualityof the received signal before buying an expensive television set.

The testing apparatus sent by Promax under reference “Prodig-2” allowsdetermining the quality of analogue signals and Digital Terrestrial TVsignals compliant with the DVB-T standard. This testing apparatusreceives signals in a frequency range comprising a plurality ofchannels. For each channel, this testing apparatus measures the level orthe channel power as well as the carrier above noise ratio C/N. Anindication that the quality of a channel is good is displayed if thelevel or power is between recommended margins and the ratio C/N isgreater than the minimum recommended value.

However, the power and the C/N ratio of a digital signal do not alwaysguarantee that the quality of the digital signal is good. Actually, echocan be present in the digital signal, due to reflections of the signalbefore reaching the antenna. In this case, a multipath channel isobtained, which comprises a plurality of similar digital signalsreceived at different times on the antenna. If these differences intimes are larger than the guard interval defined for DVB-T, the qualityof the audiovisual content may be bad. However, in this case, the poweris between recommended margins and the ratio C/N is greater than theminimum recommended value. Moreover, the same channels can be used inadjacent areas for analogue and digital signals. If the analogue anddigital signals are received on the same antenna, the quality of theaudiovisual content may be bad. However, in this case, the power is alsobetween recommended margins and the ratio C/N is greater than theminimum recommended value.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a testing apparatus in whichthe determination of the quality of the received digital signals isimproved.

To this end, the invention proposes a testing apparatus comprising meansfor receiving signals in a frequency range comprising a plurality ofchannels, means for scanning said frequency range, means for detectingat least one channel comprising a digital signal, means for measuring abit error rate of said digital signal, means for performing at least onecomparison between said bit error rate and at least one threshold, andmeans for determining a quality of said channel on the basis of saidcomparison.

According to the invention, the determination of the quality of achannel comprising a digital signal is based on a bit error rate of thisdigital signal. As a consequence, this determination is improved,because the quality of a digital signal directly depends on the detectedbit error rate.

In a preferred embodiment, the testing apparatus further comprises meansfor measuring a number of uncorrectable packets in the digital signal,said quality being further determined on the basis of said number. Thisleads to a further improvement in the determination of the quality ofthe digital signal. Actually, the bit error rate is measured during arelatively long time so that the comparison between said bit error rateand the threshold can lead to a good quality of the channel, whereas thedigital signal comprises burst errors which are not corrected and whichin fact deteriorate the resulting signal. Taking into account theseuncorrectable errors thus improves the determination of the quality ofthe digital signal.

In an advantageous embodiment of the invention, the testing apparatuscomprises means for detecting all the channels comprising a digitalsignal in the frequency range and means for determining the number ofchannels having a predetermined quality.

According to this advantageous embodiment, the testing apparatus canprovide an indication of the number of channels comprising a digitalsignal, which have, for example, a good quality. This indication isobtained without need for a user to test each channel one after theother. As a consequence, a very easy to use testing apparatus isprovided. Moreover, if the frequency range as well as the thresholds arepredetermined in the testing apparatus, the testing apparatus is eveneasier to use. Actually, a user can know the number of channels hereceives with a good quality, simply by connecting the output of hisantenna up with the testing apparatus.

The invention also relates to a method of determining a quality of atleast one channel comprising a digital signal in a frequency rangecomprising a plurality of channels, said method comprising a step ofscanning said frequency range, a step of detecting at least one channelcomprising a digital signal, a step of measuring a bit error rate ofsaid digital signal, a step of performing at least one comparisonbetween said bit error rate and at least one threshold, and a step ofdetermining a quality of said channel on the basis of said comparison.

Advantageously this method comprises a step of detecting all thechannels comprising a digital signal in the frequency range and a stepof determining the number of channels having a predetermined quality.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 a diagrammatically illustrates a frequency range comprising aplurality of channels and FIG. 1 b illustrates a detailed view of threechannels of FIG. 1 a;

FIG. 2 is a block diagram illustrating a testing apparatus in accordancewith the invention;

FIG. 3 is a block diagram illustrating the digital error corrector ofFIG. 2;

FIG. 4 illustrates a method of determining a number of channels inaccordance with an advantageous embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A frequency range comprising a plurality of channels is depicted on FIG.1 a. In this example, the frequency range comprises eleven channels 101to 111. The invention can be applied to signals in a frequency rangecomprising a different number of channels. Actually, even for the samestandard, such as the DVB-T standard, the number of channels differsfrom one geographical area to another. For example, in France, the DVB-Tsignals are broadcasted on the band 3 of the VHF frequency range (VHFstands for Very High Frequency), which is comprised between 174 and 223Megahertz, and on the band 4 and 5 of the UHF frequency range (UHFstands for Ultra High Frequency), which are comprised between 470 and862 Megahertz. The width of a channel might also differ depending on thetechnology and the geographical area. For example, in France, the lengthof a channel used for DVB-T is 8 Megahertz, but other countries canchoose 6 or 7 Megahertz.

As a consequence, it is important to notice that the invention can beapplied to a plurality of signals, as soon as these signals arecomprised in a frequency range comprising channels having apredetermined width. For example, the invention might be applied to DABsignals (DAB stands for Digital Audio Broadcasting), ATSC signals usedfor digital television in USA (ATSC stands for Advanced TelevisionSystem Committee), or ISDB-T signals used for digital television inJapan (ISDB-T stands for Integrated Services DigitalBroadcasting—Terrestrial). However, the invention cannot be applied tosatellite television using the DVB-S standard (DVB-S stands for DigitalVideo Broadcasting-Satellite), because the signals broadcasted by meansof satellite are in a frequency range which does not comprise channels.

The following description applies to television signals, in particularto digital video signals using the standard DVB-T.

In the example of FIG. 1 a, the illustrated frequency range compriseseleven channels 101 to 111. The first channel 101 does not comprise anysignal. The second channel 102 comprises an analogue television signal.The third channel 103 does not comprise any signal. The fourth channel104 comprises a digital television signal. Such a digital televisionsignal is called a multiplex in DVB-T. The fifth channel 105 comprisesan analogue signal, and so on. In FIG. 1 a, the signals arediagrammatically illustrated. FIG. 1 b gives a detailed view of measuredsignals corresponding to the fourth, fifth and sixth channels 104 to 106of FIG. 1 a.

A testing apparatus in accordance with the invention is depicted on FIG.2. Such a testing apparatus 21 comprises a tuner 210, a demodulator 211,a digital error corrector 212 and a controller 213. The testingapparatus 21 is intended to receive signals in a frequency rangecomprising channels. For example, the testing apparatus receives signalsfrom an antenna, by means of receiving means, such as a socket connectedup with the output of the antenna. In the following description, thereceived signals are the signals depicted in FIG. 1 a.

When the testing apparatus 21 receives signals, it scans the frequencyrange of the signals in order to find a multiplex, i.e. a digitalsignal. The frequency range to scan can be predefined in the testingapparatus 21. For example, the frequency range can be the range 174-862Megahertz in a testing apparatus for DVB-T intended to be used inFrance. The frequency range can also be determined by a user by means ofan interface, or by any means, such as a software application in thecontroller 213. In order to scan the frequency range channel by channel,the testing apparatus 21 has also to know the width of a channel. Thiswidth can also be predefined in the testing apparatus 21, for example 8Megahertz in a testing apparatus for DVB-T intended to be used inFrance, or can also be determined by a user by means of an interface, orby any means, such as a software application in the controller 213.

In the example of FIG. 1 a, the controller 213 knows that the testingapparatus 21 has to scan the eleven channels 101 to 111. First, thecontroller 213 sends an order to the tuner 210, so that the tuner 210converts the signal received at a frequency corresponding to a centralfrequency of the first channel 101, into a signal at an intermediatefrequency, which is the output frequency of the tuner 210. The centralfrequency of the first channel 101 can be determined easily, as thecontroller 213 knows the low frequency of the first channel 101, as wellas the width of the first channel 101.

Then, the signal at the output of the tuner 210 is demodulated by thedemodulator 211. In this example, there is no signal on the firstchannel 101, so that the demodulator 211 cannot demodulate the signal atthe output of the tuner 210. In this case, the demodulator 211 indicatesto the controller 213 that there is no demodulated signal.

Then, the controller 213 sends an order to the tuner 210, so that thetuner 210 converts the signal received at a central frequency of thesecond channel 102, into a signal at the intermediate frequency. Thecentral frequency of the second channel 102 can easily be determined, byadding the width of a channel to the central frequency of the firstchannel 101. As the signal on the second channel 102 is analogue, thedemodulator 211 cannot demodulate the signal at the output of the tuner210 and thus indicates to the controller 213 that there is nodemodulated signal.

Then, the controller 213 sends an order to the tuner 210, so that thetuner 210 converts the signal received at a central frequency of thethird channel 103, into a signal at the intermediate frequency. As thereis no signal on the third channel 103, the demodulator 211 indicates tothe controller 213 that there is no demodulated signal.

Then, the controller 214 sends an order to the tuner 210, so that thetuner 210 converts the signal received at a central frequency of thefourth channel 104, into a signal at the intermediate frequency. As thefourth channel 104 comprises a multiplex, the corresponding digitalsignal is demodulated by the demodulator 211, which indicates to thecontroller that there is a multiplex on the fourth channel 104. Thedemodulated signal is sent to the digital error corrector 212, whichdetermines a bit error rate of the digital signal on the fourth channel104. The digital error corrector will be described in more details onFIG. 3.

The bit error rate is sent to the controller 213, which compares thisbit error rate with a threshold. If the bit error rate is greater thanthis threshold, the controller 213 determines that the quality of themultiplex on the channel 104 is bad. If the bit error rate is lower thanthe threshold, the controller 213 determines that the quality of themultiplex on the channel 104 is good.

Examples of thresholds which might be used in the testing apparatus 21are 10-4 or 2.10-4. It is important to notice that a plurality ofthresholds can be used in the testing apparatus 21. For example, a firstand a second threshold can be used, leading to three different level ofquality, the first threshold being greater than the second threshold. Ifthe bit error rate is greater than the first threshold, the controller213 determines that the quality of the multiplex is bad. If the biterror rate is lower than the second threshold, the controller 213determines that the quality of the multiplex is good. If the bit errorrate is between the first and second threshold, the controller 213determines that the quality of the multiplex is medium.

The quality of the multiplex on the fourth channel 104 is then displayedon a display 22. This display 22 can be part of the testing apparatus21, or can be a separate display. In addition to the quality of thefourth channel 104, the display 22 can display further information, suchas a name of the multiplex, or the names of the channels comprised inthis multiplex. Actually, this information is transmitted with themultiplex and demodulated, so that the controller 213 can have knowledgeof this information and can order the display 22 to display it.

As a consequence, a very simple to interpret indication is displayed onthe display 22, because the quality of the multiplex is directlydisplayed, eventually together with further information on themultiplex. Hence, a user of the testing apparatus 21 does not need anyknowledge of the technology, because he immediately has access to thequality of the multiplex.

Then, the steps described hereinbefore are repeated, in order to findanother multiplex in the frequency range. These steps can be repeatedafter a certain time during which the quality of the fourth channel 104has been displayed, or after the user has required to find anothermultiplex, for example by pressing a key on an interface of the testingapparatus 21.

The demodulator 211 and the digital error corrector 212 are known fromthose skilled in the art. For example, the circuit sold by the applicantunder reference TDA10046 comprises such a demodulator and such a digitalerror corrector.

FIG. 3 illustrates the digital error corrector 212 of FIG. 2. Thedigital error corrector 212 comprises a Viterbi corrector 31, aReed-Solomon corrector 32, a first comparator 33 and a second comparator34. The encoding of digital signals for DVB-T is specified in thestandard ETSI EN 300 744. DVB-T signals comprise convolutional correctorcodes and Reed-Solomon corrector codes, which are used in order tocorrect the received signal. DVB-T data are broadcasted by packets of204 bytes, each packet comprising convolutional coding and aReed-Solomon corrector code.

When the digital error corrector 212 of FIG. 2 receives a demodulatedsignal, a first correction is performed on the packets of thisdemodulated signal, by means of the Viterbi corrector 31. Then a secondcorrection is performed on the corrected packets, by means of theReed-Solomon corrector 32. A first bit error rate BER1 is measured,which results from a comparison between the demodulated signal and thecorrected signal at the output of the Viterbi corrector 31. A second biterror rate BER2 is measured, which results from a comparison between thecorrected signal at the output of the Viterbi corrector 31 and thecorrected signal at the output of the Reed-Solomon corrector 32. The biterror rates are determined on a relatively long period of time, forexample a few hundred milliseconds.

The first and second bit error rates BER1 and BER2 can be used by thecontroller 213 in order to determine the quality of the channel, bycomparison with a threshold. However, it is preferable to use the secondbit error rate BER2, which leads to a better determination of thequality of the channel.

The digital error corrector 212 can also determine the number N ofuncorrectable packets, i.e. the number of packets which have not beencorrected during this period of time. This number N of uncorrectablepackets can be taken into account in order to determine the quality ofthe corresponding channel. Actually, it is possible that a few packetsare not corrected during said period of time, because these packets eachcomprise more erroneous bytes than the Viterbi corrector 31 and theReed-Solomon corrector 32 are able to correct. However, if the otherpackets have relatively few erroneous bytes, the second bit error rateBER2 is relatively low, because it is measured over a relatively longperiod. Hence, the controller 213 might determine that the quality ofthe channel is good, although there are uncorrectable packets, whichdeteriorate the quality of the resulting audiovisual content.

Taking into account the number N of uncorrectable packets thus improvesthe determination of the quality of the channels. For example, thecontroller 213 can determine that the quality of the channel is good ifthe bit error rate is lower than a threshold and there is nouncorrectable packets or a number of uncorrectable packets lower thananother threshold, for example 2.

FIG. 4 illustrates a method of determining a number of channels inaccordance with an advantageous embodiment of the invention. A goal ofthe advantageous embodiment of the invention is to determine the numberof channels comprising a multiplex, which have a predetermined quality.The example described hereinafter applies to the case where only onethreshold is used by the controller 213 in order to determine thequality of the channels, and the number of uncorrectable packets is nottaken into account in order to determine the quality of the channels. Ofcourse, this advantageous embodiment could be applied with a largernumber of thresholds, i.e. a larger number of quality levels and bytaking into account the number of uncorrectable packets for determiningthe quality of the channels. In the example described hereinafter, themethod is used in order to determine the number X of channels having agood quality in a predetermined frequency range.

At step 41, the number X is set to zero and the controller 213 sends anorder to the tuner 210, so that the tuner 210 converts the signalreceived at a frequency corresponding to a central frequency of thefirst channel of the frequency range, into a signal at an intermediatefrequency. This signal is demodulated, and at step 42, it is determinedif the channel comprises a multiplex or not. If yes, the bit error rateof the digital signal is measured and compared, at step 43, with athreshold. If the bit error rate of the digital signal is lower than thethreshold, indicating that the quality of the channel is good, thenumber X is incremented at step 44. If the bit error rate of the digitalsignal is larger than the threshold, indicating that the quality of thechannel is bad, the number X is not incremented at step 45. Then, it ischecked, at step 46, if the channel which quality has been determined isthe last channel in the frequency range or not. If, at step 42, thechannel does not comprise any multiplex, next step is step 46, where itis checked if the channel is the last channel in the frequency range.

If the processed channel is not the last channel, the controller 213, atstep 47, sends an order to the tuner 210, so that the tuner 210 convertsthe signal received at a frequency corresponding to a central frequencyof the next channel in the frequency range into a signal at anintermediate frequency. Then it is checked, at step 42, if the nextchannel comprises a multiplex, and so on.

If the processed channel is the last channel at step 46, then the numberX corresponds to the number of channels in the frequency range having agood quality. This number can then be sent, at step 48, to the display22 in order to be displayed. Then step 41 is performed, and a newdetermination of the number of channels having a good quality isperformed. It is advantageous to perform consecutive determinations ofthe number of channels having a good quality. Actually, when one wantsto orientate an antenna in order to obtain a large number of multiplexeshaving a good quality, the number of multiplexes having a good qualitycan depend on the orientation of the antenna. Having consecutivemeasures allows finding the best orientation for the antenna.

As a consequence, this embodiment of the invention is particularlyadvantageous, because it allows rapidly obtaining the number ofmultiplexes which are received with a good quality. Actually, such adetermination takes a few second, generally less than ten seconds.Moreover, such a determination is particularly easy, as it requires onlyconnecting the output of the antenna up with the testing apparatus, incase where the frequency range, the thresholds and the channel width arepredetermined in the testing apparatus. Hence, a user can know thenumber of multiplexes he receives, without any knowledge of thetechnology.

The display 22 can be, for example, a set of LEDs, a switched on LEDcorresponding to a channel having a good quality. In the case where morethan two levels of quality are defined, the display 22 comprises morethan one set of LEDs.

The display 22 can also be a liquid crystal display. In this case,additional information can be displayed on the display 22. Actually, thedisplay 22 can also display the names of the multiplexes having a goodquality, or the names of the channels of these multiplexes. In order toachieve this, the method described on FIG. 4 further comprises a step ofstoring information related to the multiplexes having a good quality.

An apparatus in accordance with the invention can be part of atelecommunication network comprising at least an emitter for sendingsignals, a transmission channel and a receptor for receiving saidsignals

The method of determining a quality of at least one channel comprising adigital signal in a frequency range comprising a plurality of channelsaccording to the invention might be implemented in an integratedcircuit, which is intended to be integrated in a testing apparatus. Aset of instructions that is loaded into a program memory causes theintegrated circuit to carry out the method. The set of instructions maybe stored on a data carrier such as, for example, a disk. The set ofinstructions can be read from the data carrier so as to load it into theprogram memory of the integrated circuit, which will then fulfil itsrole.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations does not exclude the presence of anyother elements besides those defined in any claim. The word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements.

1. A testing apparatus (21) comprising means for receiving signals in afrequency range comprising a plurality of channels (101-111), means forscanning said frequency range (210, 213), means for detecting at leastone channel comprising a digital signal (211), means for measuring a biterror rate of said digital signal (212), means for performing at leastone comparison between said bit error rate and at least one threshold(213), and means for determining a quality of said channel on the basisof said comparison.
 2. A testing apparatus as claimed in claim 1, saidapparatus further comprising means for measuring a number ofuncorrectable packets in the digital signal, said quality being furtherdetermined on the basis of said number.
 3. A testing apparatus asclaimed in claim 1, said apparatus comprising means for detecting allthe channels comprising a digital signal in the frequency range andmeans for determining the number of channels having a predeterminedquality.
 4. A method of determining a quality of at least one channelcomprising a digital signal in a frequency range comprising a pluralityof channels, said method comprising a step of scanning said frequencyrange, a step of detecting at least one channel comprising a digitalsignal, a step of measuring a bit error rate of said digital signal, astep of performing at least one comparison between said bit error rateand at least one threshold, and a step of determining a quality of saidchannel on the basis of said comparison.
 5. A method as claimed in claim4, said method further comprising a step of measuring a number ofuncorrectable packets in the digital signal, said quality being furtherdetermined on the basis of said number.
 6. A method as claimed in claim4, said method comprising a step of detecting all the channelscomprising a digital signal in the frequency range and a step ofdetermining the number of channels having a predetermined quality.
 7. Acomputer program comprising a set of instructions which, when loadedinto a processor or a computer, causes the processor or the computer tocarry out the method as claimed in claim
 1. 8. A telecommunicationnetwork comprising at least an emitter for sending signals, atransmission channel, a receptor for receiving said signals and atesting apparatus as claimed in claim 1.